First-principles Study On Dehydrogenation And Aromatization Mechanism Of Cycloalkene Catalyzed By GaO~+ Modified ZSM-5 In MTA Reaction

Aromatic hydrocarbons,especially benzene,toluene and xylene（BTX）,are important organic chemical raw materials.At present,aromatic hydrocarbon production technology in China relies heavily on petroleum resources.Due to China’s coal-dominated energy structure and lack of petroleum resources,it is of great significance to develop aromatics preparation processes independent of petroleum resources in order to solve the contradiction between the rapid growth of the demand for these chemicals and the exhaustion of petroleum reserves.Methanol to aromatics（MTA）process is considered as a promising alternative process for the production of BTX and other light aromatics,which has attracted extensive attention from academia and industry.ZSM-5 zeolite with MFI topological structure is the most widely used catalyst in MTA reaction because of its unique 10-member ring pore structure,which is conducive to the shape selection of light aromatic hydrocarbons.However,the low aromatic yield and poor selectivity of BTX limit its practical application.Studies in recent decades show that metal-modified zeolites can significantly improve aromatic selectivity and reactivity in MTA reactions.Deng et al.recently found that the reactivity of cycloolefin on Ga O⁺modified ZSM-5zeolites was much higher than that on H-ZSM-5.These results confirmed that Ga O⁺with adjacent B acids formed Lewis acid Sites（LAS）-Br（?）nsted acid Sites（BAS）acid pairs,which promoted dehydrogenation and inhibited hydrogen transfer process,thus effectively improving aromatic hydrocarbon yield.In order to further clarify the effect of metal-modified zeolites on dehydrogenation aromatization,we calculated the possible positions of Ga O⁺based on the spatial proximity of Lewis acid and Br（?）nsted acid to form LAS-BAS acid pairs by using density functional theory（DFT）method including BEEF-vd W correction.The dehydrogenation and aromatization of methylcyclopentene and cyclohexene in H-ZSM-5 and Ga/ZSM-5 zeolites were systematically studied,and the micromechanism of the reaction was elucidated.The dehydrogenation and aromatization activities of H-ZSM-5 and Ga/ZSM-5 at 623 K were analyzed using the energy span model.This study revealed the synergistic effect of Ga O⁺and B acid on the olefin aromatization activity of ZSM-5 zeolites at the zeolites,which can provide a theoretical basis for the optimal design of catalysts.The calculation results indicate that:1.In H-ZSM-5,the dehydrogenation and aromatization of methylcyclopentene goes through dehydrogenation and deprotonation steps,as well as hydrogen transfer and ring expansion,and then deprotonation to produce benzene.The energy barriers of two dehydrogenation reactions of methylcyclopentene are188 k J/mol and 220 k J/mol respectively,while hydrogen transfer and ring expansion reactions are more likely to occur.The dehydrogenation of cyclohexene is directly catalyzed by B acid,which directly binds with H on cyclohexene to form H₂.Benzene was formed by dehydrogenation at 623 K in two steps,overcoming energy barriers of 169 k J/mol and 310 k J/mol,respectively.2.In Ga/ZSM-5,due to the spatial proximity of Ga O⁺and B acid,LAS-BAS acid pairs were formed,which enhanced the acidity of B acid.Under the synergistic action of LAS-BAS acid pairs,Firstly Ga O⁺activates C-H bond of cycloolefin to form Ga（H）O,and then H in Ga（H）O is removed together with B acid proton H to form H₂.The dehydrogenation of methylcyclopentene and cyclohexene to H₂ is a two-step reaction process,which is different from the direct dehydrogenation mechanism in H-ZSM-5.The effective barriers for the dehydrogenation of methylcyclopentene are 121 k J/mol and 133 k J/mol,respectively.The effective barrier for dehydrogenation of cyclohexene is 152k J/mol and 153 k J/mol,respectively.Compared with the dehydrogenation barrier of H-ZSM-5,it was found that the dehydrogenation barrier of cycloolefin was mainly reduced by Ga O⁺modified ZSM-5 zeolite.The reaction barrier of ring expansion is 223 k J/mol,which is higher than that of H-ZSM-5.3.Using the energetic span model,the total energy barrier of the reaction of cyclohexene and methylcyclopentene in H-ZSM-5 was determined to be 333k J/mol and 236 k J/mol,respectively,at 623 K,indicating that methyl cyclopentene contributed more to the yield of aromatic hydrocarbons in H-ZSM-5.Under the synergistic action of LAS-BAS in Ga/ZSM-5,the total energy barriers of the aromatization of methylcyclopentene and cyclohexene are223 k J/mol and 200 k J/mol,respectively.It can be seen that cyclohexene contributes more to the increase of aromatic hydrocarbon yield.By comparing the total energy barrier of dehydrogenation of cyclohexene and methylcyclopentene between H-ZSM-5 and Ga/ZSM-5,it can be seen that Ga O⁺modified ZSM-5 zeolites significantly improve the dehydrogenation activity of cycloolefin and promote the conversion of olefins to aromatic hydrocarbons.